mixing transmission fluid types

hey guys

had the motor and trans out the truck, did a little work to the trans, put it all back together and added fluid

The local dodge dealership told me to use ATF+4 and thats what I used to fill it back up to full - it used about 4 litres to do so

I went back to Napa to get 1 more litre and all they had was ATF+3 and was told the only difference was additives - and ATF 2,3&4 were compatable. I recently read that ATF+4 is synthetic? Is this true?

I don't know exactly what was in it for the last umpteen years - but I would have bought the fluid from chrysler.

anyway- im now doubting what I did - would it hurt to mix them?

(it's a 1979 727 loadflight behind an summertime street driven 325 horse 360)